A flexographic printing process conventionally employs rollers to distribute a water-soluable ink to the printing dies. The ink is introduced into a fountain formed by the trough between two parallel touching rollers. Conventionally, the surface of one of the rollers is finely engraved for transferring the ink to a printing die. The mating roller of said pair of rollers is a rubber or plastic covered roller so that pressure may be exerted against the engraved surface to remove surplus ink.
It is highly important that there always be ink in a sufficient quantity in the fountain to maintain lubrication between the rollers while the rollers are rotating. If no ink were present in any part of the fountain between the rollers, the different surface speeds of the rollers would cause rapid abrasion of the rubber or plastic surface on the metering roller and ultimately damage to the surface of the engraved roller.
One use of a flexographic printing apparatus is the printing of sheets of corrugated paperboard in the manufacture of cartons. Printing presses for such manufacture usually include other operations such as scoring, slotting, and die cutting, all of which generate undesirable dust. To keep the printing rollers clean and prevent contamination of the ink, the printing section of the press is shrouded. Such shrouding prevents the introduction of dust but also prevents the operator from observing the printing rollers for determination that sufficient ink is present.
Conventionally, the ink is pumped to the ink fountain from a supply pail. Should the ink supply diminish below a predetermined amount, the operator may not be aware of the same until notice is taken of the off-color of ink on the sheets of paperboard. Then, the press is stopped to permit immediate replenishment of the ink supply. If only a narrow small printing die is in use, the ends of the rollers can be dry and without ink whereby the rollers may be damaged.
There is a need for a simple means to warn the operator that the ink level in the ink fountain is too low. The ink level cannot readily be measured with a float because the depth of the ink is shallow and there is a danger that the float will be drawn into the nip of the rollers. Other liquid level sensing devices have been tried but have not been found to be satisfactory.
This invention relates to apparatus including a transfer roll and a metering roll. The rolls cooperate to define a trough therebetween along the length thereof for receiving a liquid printing medium such as flexographic ink. Motor means are provided for rotating said rolls about their longitudinal axes but in opposite directions. The printing medium acts also as a lubricant between contacting peripheral portions of said rolls.
An alarm means is provided together with a means for triggering the alarm means in response to the reaction torque of the motor drive means on the metering roll. The alarm means is triggered when the lubricity of printing medium in said trough is below a predetermined level.
In the preferred embodiment of the present invention, the level of printing medium such as ink is not measured directly but is sensed by a means which responds to a condition of rotation of the metering roll. When the ink is the trough is below a predetermined level, the ink no longer functions as a lubricant along the entire length of the metering roll and transfer roll. Then, since the two rolls are independently driven and the transfer roll conventionally has a higher surface speed, the increased friction affects the torque required to drive the metering roll. The transfer roll then tends to drive the metering roll. This change in the torque applied to the metering roll is sensed and an alarm is triggered.
It is an object of the present invention to provide a simple warning apparatus to alert a printing press operator that the ink level in the fountain is too low.
It is another object of the present invention to provide apparatus by which a printing press can be caused to automatically stop before fountain rolls are damaged as a result of an insufficient supply of ink.
Other objects will appear hereinafter.